The invention relates generally to a current limiting circuit breaker, and more particularly, to a bridge contact structure and operating means for the circuit breaker.
Circuit breakers are widely used to provide protection for electrical distribution systems against damage caused by overload or fault current conditions. Over the years, as the capacity of power sources increased, it became necessary to provide increased interrupting capability for circuit breakers to adequately protect an electrical distribution system. To provide this level of protection in an economical manner, current limiting circuit interrupters were developed to limit the amount of fault current to a level substantially below that which the current source was capable of supplying.
Typically, circuit breakers require a certain contact closing force to reduce resistance between the contacts and to reduce the resistance heating generated during normal closed circuit conditions in order to meet required temperature restrictions. This contact force is most commonly obtained by means of extension or compression springs attached to the contact arm or arranged to exert the force on the contact arm. The higher the current rating of the circuit breaker, generally the greater the required contact force. In a current limiting circuit breaker, the contact arms separate independently of other portions of the operating mechanism to produce the current limiting action and, in the process, stretch or compress the springs from their normal positions. The resistive force supplied by these springs during current limiting operation thus significantly reduces the acceleration of the contact arms and the degree of current limiting. This is especially true with high current circuit breaker ratings. Accordingly, it would be appreciated that it would be highly desireable to minimize the contact spring force in order to produce maximum acceleration of the contact arm during blow-off. At the same time, however, sufficient contact closing force during normal closed circuit conditions must be maintained to reduce resistance heating of the circuit breaker contacts.
U.S. Pat. No. 4,409,573, which issued on Oct. 11, 1983 to Bernard DiMarco and Andrew J. Kralik, discloses a circuit breaker with a current limiting feature provided. The current limiting contacts blow open in response to fault current and are latched in the open position. The breaker is then reset by use of the operating handle. While this breaker achieves a current limiting effect, a higher current rating can be achieved by using blow open contacts of the current limiting type in series with this breaker. This configuration is disclosed in U.S. Pat. No. 4,458,224, which issued on July 3, 1984 to Bernard DiMarco and Andrew J. Kralik. In this embodiment, current limiting blow open contacts are placed in series with the circuit breaker. The blow open contacts are configured to reclose automatically by the action of biasing springs which also function to give the required closed contact pressure. It is apparent that the blow open force is a function of the current magnitude and the length of the parallel conducting paths which create the blow open force. The blow open force in this configuration is thus limited by the physical requirements of the circuit breaker enclosure. Accordingly, it would be appreciated that it would be highly desirable to provide increased blow open force for more rapid separation of the contacts due to a fault without increasing the physical dimensions of the circuit breaker enclosure.
U.S. Pat. No. 3,991,391, which issued on Nov. 9, 1976 to John A. Wafer, and U.S. Pat. No. 4,132,968, which issued on Jan. 2, 1979 to Walter W. Lane, disclose a current limiting circuit breaker which has a slot motor magnetic drive device. In this construction, the threshold level of overload current which produces current limiting action is raised, while the degree of current limiting action during high overload currents is maintained by placing a thin saturable magnetic steel plate across the open end of the slot motor magnetic drive device. During over current conditions below the threshold value, the plate shunts most of the magnetic flux and prevents production of magnetodynamic force upon the contact arm. Above the threshold level, the over current generates magnetic flux sufficient to saturate the plate and force additional flux into the air gap where the flux interacts with the contact arm to drive the contact arm into the slot and produce current limiting action in a normal manner. This configuration changes the normal response to a low level fault which the normal circuit breaker mechanism can handle and thereby limits the over current response of the current limiting contacts. Accordingly, it will be appreciated that it would be highly desirable to have a current limiting circuit breaker which responds rapidly to low level as well as high level faults.
It is apparent that rapid opening of the contacts is essential to successful operation and longevity of the current limiting contacts. For a given current, the blow open forces can be effectively increased by lowering the closing force of the contacts which is not really desired because closing contact pressure must be retained or by increasing the magnetic field.
U.S. Pat. No. 4,001,738, which issued Jan. 4, 1977, to Claude Terracol and Pierre Schueller, discloses a circuit interrupter having an electromagnetic repulsion device. In this configuration, a circuit interrupter has a magnetic circuit energized by the current flowing through the interrupter and an induction plate that is movable with the movable contact of the interrupter. The abrupt rising of a fault current induces secondary currents in the induction plate which is located in the air gap of the magnetic circuit as long as the interrupter is in the closed circuit position. The secondary currents tend to expel the induction plate from the air gap thereby moving the movable contact vigorously away from the magnetic circuit. This increases the repulsing forces for a given current thereby ensuring fast opening operation. An alternate embodiment discloses contacts which form a two-loop current path. That is, a path in which current enters one conductor, flowing in a first direction, then flows through the movable contact in the opposite direction and then flows through the second stationary conductor in the first direction. This two-loop configuration effectively doubles the magnetic repulsion force. U.S. Pat. No. 4,118,681, which issued Oct. 3, 1978 to Jean Pierre Nebon and Robert Morel also discloses a circuit breaker having a two-loop blow off configuration. This patent also discloses a retarding member which is mechanically linked to the movable contact assembly to delay the reclosing of the contact and to prevent a reclosing before tripping of the circuit breaker. While the circuit breakers disclosed offer fast operation in response to a high level fault condition, there is still needed a circuit breaker which opens quickly and cleanly in response to a low level fault condition. Accordingly, it will be appreciated that it would be highly desirable to provide current limiting circuit breaker contacts which cleanly open in response to low level fault conditions. Ideally, such contacts will snap open.
It is an object of the present invention to provide a current limiting circuit breaker which limits the current to a preselected maximum value.
Another object of the present invention is to provide a current limiting circuit breaker which opens quickly and cleanly in response to a low level fault condition.
Yet another object of the present invention is to provide current limiting contacts which snap open in response to a low fault condition.
Still another object of the present invention is to mechanically reduce the biasing force on the contact bridge in response to a preselected amount of movement of the contact bridge.